I read that for gnome should install gnome-session and gnome-desktop. I found gnome-desktop in ports but gnome-session not found (ftp://ftp.openbsd.org/pub/OpenBSD/4.6/ports.tar.gz). What to do about gnome-session ? Is there a possibility to determine which additional ports will be installed because i don't want full gnome?

Gnome session will get you a minimal, but working gnome desktop. In /usr/ports/gnome/session/files you will find a text message from the Gnome OpenBSD maintainer that tells you how to add additional packages and otherwise tweak the installation.

You may also want to consider installing packages rather than building from ports. In OpenBSD the end result is the same ie. the default port install builds the same binary package that is installed from packages. If you have very specific needs you can build from ports although it will take you alot longer.